Composite materials often have better mechanical properties than either the matrix or the dispersates alone. The desirable properties are generally maximized when the dispersates are distributed as uniformly as possible within the matrix, but making composites with such uniform dispersions has often proved difficult.
One method for making composites is to disperse the dispersates in a precursor liquid and then form the composite by solidifying the liquid part (or "dispersion medium") of the dispersion. A significant practical difficulty with this simple method is that any difference in density between the dispersates and the dispersion medium may cause a generally undesired segregation of the dispersates toward either the top or bottom of the dispersion and thus of the final composite.
U.S. Pat. No. 4,735,656 of Apr. 5, 1988 to Schaefer et al. teaches a method of avoiding segregation due to density differences by mixing metal particulates with ceramic particulates, heating the mixture to a temperature sufficient to cause partial melting of the metal so that it fuses into a dense matrix when cooled, but insufficient to cause the ceramic particulates to float in the metal matrix.
Another frequent problem with simple mixing is that some of the most desirable composites are made from dispersates that are difficult to wet by any known fluid precursor of the desired matrix phase. International Patent Application WO 87/06624, published Nov. 5, 1987, teaches a method of ameliorating the difficulties when using dispersates that are difficult to wet, by using specific types of dispersing and/or sweeping impellers that promote high shear mixing while minimizing the introduction of gas into the mixture and the retention of gas at the interface between the dispersates and the dispersion medium. U.S. Pat. No. 4,662,429 of May 5, 1987 to Wada et al. teaches use of lithium in a melt of aluminum matrix alloy to facilitate wetting of the reinforcing material and ready dispersal thereof in the matrix alloy. European Patent Application No. 87 201 512.8 published Feb. 24, 1988 describes composites of a zinc-aluminum alloy reinforced with silicon carbide powder which has good mechanical properties without the difficulties often experienced with other similar composites.
Difficulties in making good quality composites, and various expedients tried to overcome those difficulties are generally reviewed by P. K. Rohatgi et al. in "Solidification, structures, and properties of cast metal-ceramic particle composites", 31 International Metals Reviews 115-39 (1986). One method from Rohatgi is described in more detail by B. C. Pai et al., 13 Journal of Materials Science 329-35 (1978). This method involves pressing together dispersates with powdered matrix material to form a pellet, introducing this pellet beneath the surface of a quantity of fluid matrix precursor material for long enough to melt the pellet matrix, stirring to disperse the dispersates within the total amount of fluid precursor material, and then solidifying the dispersion. Analogously, J. Cisse et al. in 6B Metallurgical Transactions 195-97 (1975) describe use of a "master alloy" of rods made from sintered aluminum powder and containing 10% of aluminum oxide.
All the prior art methods known to applicants initially produce composites with substantial porosity unless the dispersates are quite easily wet by the fluid matrix material, and in the latter case, the properties of the composite are often degraded by chemical reaction between the matrix and the dispersates. It is therefore an object of this invention to produce composites as free as possible from porosity and to minimize the time required to make the composite, so that chemical degradation of the interfaces in the composite is minimized.